Wireless sensors have a great deal of potential in numerous applications where a wired readout is difficult, for example, due to harsh operating conditions, rotating parts or cost and complexity of wiring. Wireless sensors are passive, battery-assisted semi-passive, or active. The advantages of passive sensors are that their life-time or operation conditions are not limited by the battery and that they are inexpensive. Examples of passive wireless sensors are silicon-based radio frequency identification (RFID) tags, surface acoustic wave (SAW) RFID, and inductively-coupled resonance-circuit sensors.
The RFID tags are based on integrated circuits (ICs), which enable several sophisticated features such as rewritable memory and anti-collision protocols. The cost of an RFID tag is low due to their high manufacturing volumes. RFID is mostly used for identification but can also be used to realize other sensors by adding a sensing element to the tag. Passive RFID have some disadvantages. The power rectifier that generates the required power for the IC limits the largest read-out distance and the highest operation frequency.
The highest operation frequency and read-out distance of RFID are limited by the rectified power for the IC and are a few GHz and 5-10 m, respectively. An additional sensor element further increases power consumption. Therefore, RFID does not suit well for applications where a long distance or a high operation frequency is required. High operation frequencies are needed to enable small tag sizes and accurate spatial localization of the tag.
SAW tags transform the electromagnetic energy to SAWs propagating on a piezoelectric substrate. The SAWs are then manipulated and transformed back to electromagnetic waves. The SAW tags lend themselves well as sensors as the propagation properties of SAWs can be tailored to be sensitive to several measured quantities, such as temperature or strain, and no external sensor element is necessarily needed, although it is possible to use one. The highest operation frequency is typically limited to a few GHz by the line width of acoustical reflectors fabricated on the substrate. In addition, a piezoelectric material as a sensing element may limit their applications.
Inductively-coupled resonance-circuit sensors are utilized, for example, to measure strain and moisture. These sensors consist, comprise, or comprise substantially of a simple electrical resonance circuit, whose resonance frequency is sensitive to the measured quantity. The simple sensor structure enables a low manufacturing cost. However, these sensors can not be read across large distances as they require near-field coupling to the reader device.
Mixer sensors contain mixing elements such as diodes and transmit the sensor data either at a harmonic or intermodulation frequency when illuminated by a reader device. Although microwave illumination is usually used, an optical excitation signal can also be used for improved spatial localization.
Harmonic radar and tags were first proposed for traffic applications and were used for tracking insects and avalanche victims. The intermodulation principle was first proposed for telemetry and later was used to implement wireless ferroelectric temperature sensor and was modified for wireless MEMS sensors based on mechanical mixing. The advantage of the intermodulation principle over the harmonic scheme is a smaller frequency offset which facilitates circuit design and compliance with frequency regulations. Generally, harmonic and intermodulation sensors can use a very high frequency and be operated at a large distance.
Passive wireless sensors can also be implemented with the MEMS technology. The MEMS sensor, when illuminated with two different frequencies, replies the sensor data at an intermodulation frequency. The intermodulation interrogation concept is somewhat similar to the harmonic radar.
Intermodulation sensors utilize mixing elements (a ferroelectric varactor and MEMS resonator) as sensing elements and do not facilitate a sensing element in a straightforward way. In addition, proximity of dielectric or conductive material distorts the read-out of the ferroelectric intermodulation sensor.
Identification RFID tags are limited to small frequencies and relatively short read out distances and are not well-suited for harsh environments (high temperature etc.).
SAW-solutions suit only for certain measurement quantities (temperature, strain) and are limited to frequencies below a few GHz. Typically they are also rather expensive (made on piezo-electric substrates).
Interrogation of resonance sensors requires a near-field coupling and these sensors are therefore limited to very short read-out distances. These sensors also require more complex read-out technique as the coupling between the sensor and the read-out unit affects the sensor.